Cycloaddition versus Cleavage of the C=S Bond of Isothiocyanates Promoted by Digallane Compounds with Noninnocent α-Diimine Ligands

Whereas the chemistry of single-bond activation by compounds of the main group elements has undergone some development in recent years, the cleavage of multiple bonds remains underexplored. Herein, the reactions of two digallanes bearing α-diimine ligands, namely, [L¹Ga−GaL¹] (1, L¹=dpp-dad=[(2,6-iPr₂C₆H₃)NC(CH₃)]₂) and [L²Ga−GaL²] (2, L²=dpp-bian=1,2-[(2,6-iPr₂C₆H₃)NC]₂C₁₀H₆), with isothiocyanates are reported. Reactions of 1 or 2 with isothiocyanates in 1:2 molar ratio proceeded with [2+4] cycloaddition of the C=S bond across the C₂N₂Ga metallacycle with formation of C−C and S−Ga single bonds to afford [L¹(RN=C−S)Ga−Ga(S−C=NR)L¹] (3, R=Me; 4, R=Ph) and [L²(RN=C−S)Ga−Ga(S−C=NR)L²] (8, R=allyl; 9, R=Ph). In the cases of 8 and 9, this cycloaddition is reversible. The digallanes reacted with 2 equiv of PhNCS in the presence of Na metal or at high temperatures through a unique reductive cleavage of the C=S bond to yield the disulfide-bridged digallium species [Na(THF)₃]₂[L¹Ga(μ-S)₂GaL¹] (5), [L²Ga(μ-S)₂GaL²] (10), and [Na(DME)₃][L²Ga(μ-S)₂GaL²] (11). Moreover, products 4 and 5 can further react with an excess of isothiocyanate, through cleavage of the C=S bond or cycloaddition, to give the bis- or mono-S-bridged complexes [Na(THF)₂]₂[L¹(PhN=C−S)Ga(μ-S)₂Ga(S−C=NPh)L¹] (6) and [L¹(PhN=C−S)Ga(μ-S)Ga(S−C=NPh)L¹] (7). All the newly prepared compounds were characterized by elemental analysis, single-crystal X-ray diffraction, IR spectroscopy, NMR (3–9) or ESR spectroscopy (11), and DFT calculations.